Recently, in order to increase the protection and aesthetic appearance of a display in a flat panel display device such as mobile phone and personal digital assistance (PDA), a thin plate-shaped cover glass having a region wider than an image display portion is provided on the front surface of a display. Reduction in the weight and thickness is required for such a flat panel display device, and to meet this requirement, the cover glass used for protecting a display is also required to achieve reduction in its thickness. However, if the thickness of the cover glass is reduced, the strength decreases, and there arises a problem that the cover glass itself may be cracked by the falling or the like during use or carrying and the primary role of protecting a display device may not be fulfilled.
For this reason, in the conventional cover glass, the flaw resistance of the cover glass has been increased by chemically strengthening a glass plate to thereby form a compressive stress layer on the surface (for example, Patent Document 1).
However, if the cover glass encounters an impact as in the case where the user drops a flat panel display device by mistake, a slow crack that is a crack growing in the glass at a relatively slow rate from a flaw penetrating a compressive stress layer is sometimes initiated even in a chemically strengthened cover glass (hereinafter, such a cracking manner of glass is referred to as “slow cracking”).